Ask the Stamping Expert: How can stampers maintain proper stripper clamp force?

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10 Haz 2020
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Q: We are experiencing a stripping problem with our single-hit piercing tool. The tool construction is a basic three-plate design. The die chase has sixteen 1/4-in.-dia. holes in a 4- by 4-in. array. We are piercing a 7-mm-thick aluminum plate in one step. The stripper has 10 springs providing 15% of the needed punching force.

After piercing is complete and the press begins to retract, the aluminum plate sticks to the punches. The spring-loaded stripper does not strip the raw material off the punches as the tool moves up off dead bottom of the stroke. There is a delay and then a severe snap, resulting in a heavy bang that occurs at some point during the upward cycle of the press as the stripper finally strips the material off the punches. We added four spring pins to eject the plate off the face of the stripper, but this did not help.

We reengineered the punch-to-die clearance from 3% to 6%, because the fracture plane of the slug was irregular and showed signs of secondary shear. This improved the fracture plane a bit but did not solve the problem. For design reasons, it is impossible to add more springs to the stripper.

Should we increase die clearance again? What is the correlation between die clearance and stripping force?

A: This is not an uncommon problem, and you should give it ample consideration during tooling design phase. Always calculate the theoretical spring pressure required; double it when using the stripper for clamping during cutting and triple it when using the stripper to clamp the material to prevent movement during forming. One of the common causes of dimensional variation when blanking and forming in large, complex progressive dies is the lack of sufficient spring pressure to prevent unwanted raw material and stripper movement while work is being done.

This is one of the reasons I am such a big fan of using nitrogen springs in progressive tooling. In recent years they have become very reliable, and generally you can get two to three times the spring force in about the same space as a coil spring. In addition, we have been able to run them at speeds well over 600 strokes per minute.

The example you have asked about is fairly straightforward. Your tool design does not have enough spring pressure to overcome your stripping force. It’s hard to calculate this exactly because, as you noted, the die clearance affects it. (The tighter the clearance, the greater the stripping force required.) The lubrication, tooling surface finish, and raw material properties also affect stripping force. But there are textbook formulas for calculating cutting and stripping forces:

  • Cutting force = Periphery length x Sheet thickness x Shear strength
  • Stripping force = 10% to 20% of cutting force
(After calculating stripping force, add a minimum 30% safety factor)

Provided no shear and break requirements are spelled out on the part print, there are a couple ways to ensure you have the proper punch-to-die clearance:

  • If the pierced hole is a few tenths of an inch smaller than the punch size, the clearance is too tight. If the hole is bigger, the clearance is too high.
  • If you can measure a slight bulging or increase in material thickness around the trimmed opening in the progressive die strip, the punch-to-die clearance may be too tight and the stripper spring pressure too light, which allow the material to bulge during piercing.
 
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